The caves are kitted out with modern sound and lighting systems. Areas of the facilities that would have been actively used in plutonium processing are lit in an eerie green light that wouldn't look out of place in a science fiction movie.

The soldiers who dug out the enormous caves worked in dangerous conditions, tunnelling into the rock with small drills, shovels and dynamite. It was a dangerous job, but workers were paid the equivalent of around £1 ($2.44) per month.

Officially the death toll stands at around 100, but even a tour guide who showed a New York Times journalist around the plant doesn't believe that figure. "The environment was too harsh," she said.

“Ultimately we worked on the project because we thought we were working for the nation," one former soldier revealed. He worked on the site from 1969 to 1974. "If we knew that in the end it would be made into a tourist site, we never would have participated.”

Step inside the world’s largest nuclear bunkers